Within the next 10 years, more than 70 million people are going to join the ranks of seniors. In an aging population, the articular cartilage that allows bones to smoothly move over each other wears down with time and unlike many tissues in the body, articular cartilage cannot heal itself. At this time, options that help to relieve severe degenerative joint pain, or osteoarthritis, include joint replacement or fusion. As examples, approximately 200,000 total knee joint and over 300,000 hip joint replacement operations are performed annually, and typically these artificial joints last about 10 to 15 years. Chronic lower back pain also affects both workforce productivity and health care expense, and there are currently over 500,000 surgical procedures performed annually in the United States in an attempt to alleviate lower back pain that persists following failure of more conservative therapy (e.g., bed rest, pain and muscle relaxant medication, physical therapy or steroid injection). The source of this pain may originate from dysfunction among a plurality of anatomical structures (as will be shown and described below) that are comprised in the spine, including facet joints.
Facet joints can become arthritic due to degeneration with aging, trauma, or disease (e.g., pathologies that include inflammatory, metabolic, or synovial, disorders). In addition, fractures, torn ligaments, and disc problems (e.g., dehydration or herniation) can all cause abnormal movement and alignment, putting extra stress on the surfaces of the facet joints.
The physiological response to this extra pressure is the development of osteophytes, i.e., bone spurs. As the spurs form around the edges of the facet joints, the joints become enlarged, a condition called hypertrophy, and eventually the joint surfaces become arthritic. When the articular cartilage degenerates, or wears away, the bone underneath is uncovered and rubs against bone. The joint becomes inflamed, swollen, and painful.
Facet joint arthritis is a significant source of neck and back pain, and attributable to about 15 percent of persistent lower back pain. Upon failure of conservative treatment for facet joint pain, such as intra-articular steroid/local anesthetic injections administered under fluoroscopic guidance, some patients with chronic pain may eventually require surgical intervention for facet joint arthritis including, for example, facet rhizotomy; facetectomy to remove the facet joint to reduce pressure on the exiting nerve root; total joint replacement, or facet arthrodesis (i.e., fixation leading to fusion, where the two articulating surfaces of the joint grow solidly together and form a single, solid piece of bone). However, as will be noted in more detail below, while these surgical procedures may alleviate back pain, many joint replacements and all fusions do not restore the normal physiological function and motion attributable to healthy anatomical form. Rather, they often significantly alter spinal biomechanics which can cause or exacerbate co-existing spinal instabilities and degeneration at other spinal levels or in other joints associated with spinal motion.
To understand spinal biomechanics, and the impacts of dysfunction and therapy, it is perhaps useful to first consider the spinal anatomy. The vertebrae of the spinal cord are conventionally subdivided into several sections. Moving from the head (cephalad) to the tailbone (caudal), the sections are cervical, thoracic, lumbar, sacral, and coccygeal. Regardless of location, each vertebra forms two pedicles and two laminae that combine to define a spinal foramen in which the spinal cord is protected. Extending from the pedicles are two transverse processes. Extending from the midline of the vertebra where the two laminae meet is a spinous process. These three processes serve as a connection point for ligaments and muscles. Adjacent vertebrae are separated by an intervertebral disc, and surfaces of the adjacent vertebrae form portions of two fact joints by and between the two vertebrae (it being understood that relative to a spinal segment consisting of an intermediate vertebra, an adjacent cephalad vertebra, and an adjacent caudal vertebra, the intermediate vertebra forms portions of four facet joints, two facet joints with the cephalad vertebra and two facet joints with the cephalad vertebra).
As illustrated in FIG. 1, a facet joint 20 is composed of a superior facet 22 and an inferior facet 24. The superior facet 22 is formed by the vertebral level below the intervertebral disc (a superior articular facet faces upward from the junction of the lateral mass and the pedicle) and the inferior facet 24 is formed by the vertebral level above the intervertebral disc (an inferior articular facet, which faces downward). On the superior articular facet 22 is a superior articulating surface, and on the inferior articular facet 24 is an inferior articulating surface. Facet joints are oriented obliquely to the sagittal plane, and the joint space itself is curved from front to back. The more posteriorly located inferior facet 24 is convex, whereas the more anteriorly located superior facet 22 is concave. There are two facet joints between each pair of vertebrae, one on each side, from the top and bottom of each vertebra. The joints combine with the disc space to create a three-joint complex at each vertebral level, and each joint extends and overlaps neighboring vertebral facet joints, linking each other and hence the vertebrae together. The facet joints are synovial joints; each joint including two opposing bony surfaces with cartilage 26 between them and a capsule 28 around the joint. More specifically, synovial fluid 30 is contained inside the joint by the joint capsule 28, a watertight sac of soft tissue and ligaments that fully surrounds and encloses the joint, which keeps the joint surfaces lubricated. The ends of the bones 22, 24 that make up the synovial facet joint 20 are normally covered with articular, hyaline cartilage that allows the bones to glide against one another providing the flexibility that allows the movement of vertebral bodies relative to one another.
The assembly of two vertebral bodies, the interposed spinal disc and the attached ligaments, muscles and facet joints (the inferior articular processes that articulate with the superior articular processes of the next succeeding vertebra in the caudal direction) is referred to as a “spinal motion segment.” Each motion segment contributes to the overall flexibility of the spine and contributes to the overall ability of the spine to provide support for the movement of the trunk and head, and in particular, the facet joints enable torsional (twisting) stability. When the facets of one or more vertebral bodies degenerate, or otherwise become damaged such that the vertebrae no longer articulate or properly align with each other, there is a resulting loss of mobility, and pain or discomfort. The functional role of facet joints in a spinal motion segment is thus relevant to an understanding of the operative and functional advantages of the facet joint methods and devices disclosed herein, which achieve dynamic stabilization and mobility preservation without constraining motion in any plane.
In the context of the present disclosure, “dynamic” refers to non-static devices with an inherent ability to allow mobility by enabling or facilitating force or load bearing that assist or substitute for physiological structures that are otherwise compromised, weakened or absent. The spinal motion preservation assemblies of the present disclosure provide dynamic stabilization across a progression-of-treatment interventions for treating symptomatic joint pain.
As indicated above, facet joints are located on the posterior column of the spine. In the context of this discussion: anterior refers to in front of the spinal column, and posterior refers to behind the column; cephalad means towards a patient's head (sometimes “superior”); caudal (sometimes “inferior”) refers to the direction or location that is closer to a patient's feet. As the present application contemplates accessing various vertebral elements and joints through a preferred approach that comes in from a percutaneous posterior approach, proximal and distal are defined in context of this channel of approach. Consequently, proximal is closer to the beginning of the channel and thus closer to the clinician, distal is farther from the beginning of the channel and thus more distant from the clinician. When referencing access and delivery tools, distal would be the end intended for insertion into the access channel, and proximal refers to the opposing end, generally the end closer to the handle for the delivery tool. When referencing implants, generally distal would be the leading end first inserted into the joint, and proximal refers to the trailing end, generally the end in engagement with the deployment tool.
There is a cause-and-effect relationship among intervertebral disc integrity, facet loads and spinal degeneration. Specifically, the progressive loss of disc height with disc degeneration often also alters the facet joint's mechanical ability as the facet joints may degenerate or dislocate, and the physiologic ligaments lose elasticity and their load carrying ability. More specifically, with disc-space narrowing, as frequently occurs with degenerative disc disease, there is increased load in the facet joints, especially in extension, and concomitant degeneration of the facet joints and capsules. Since the facet joint capsules are primarily loaded in flexion and in rotation, and the facet joints are the primary resistors against rotational or torsional forces (e.g., normally the facets control approximately 30% of axial rotation), facet joint degeneration significantly alters spinal mobility.
The need to provide minimally invasive therapies that provide pain relief while restoring and preserving the biomechanical function of the physiologic facet joints is paramount to overall spinal mobility, and to date, therapies have not adequately satisfied all of these issues, as noted below.
One therapy, facet rhizotomy, involves techniques that sever small nerves that go to the facet joint. The intent of the procedure is to stop the transmission of pain impulses along this nerve. The nerve is identified using a diagnostic injection. Then the surgeon inserts a large, hollow needle through the tissues in the low back. A radiofrequency probe is inserted through the needle, and a fluoroscope is used to guide the probe toward the nerve. The probe is slowly heated until the nerve is severed. Another technique using pulsed radiofrequency does not actually burn the nerve, rather, it is believed to stun the nerve. Yet another technique involves de-enervation by probe tip freezing, and still another procedure involves carefully controlled injection of botox toxin to treat muscle spasm, a protective reflex that may occur when the facets are inflamed which causes the nearby muscles that parallel the spine to go into spasm. While these procedures may provide pain relief, they do not address ongoing joint degeneration, e.g., wear on articulating surfaces, which leads to kinematic and biomechanical dysfunction that may in turn lead to transition syndrome (i.e., progression of degeneration and pain in other joints) at other levels.
While certain clinicians have advocated prosthetic total joint replacement of damaged facet joints, in practice it is difficult to implement a prosthetic facet joint for a variety of reasons including the variability in facet joint geometry from facet joint to facet joint and the high level of interaction between the facet joint and the other components in the spinal column. Moreover, joint replacement is an invasive and time-consuming procedure, requiring pre-preparation of joint surfaces and removal of bone, and thus there are associated risks, including blood loss and morbidity, increased anesthesia time, and increased convalescence time.
Another therapeutic treatment of the facet joint is to affix the superior articular process to the inferior articular process using a facet screw. Although this fixation therapy may alleviate symptoms associated with a degenerated facet joint, it also sacrifices some of the ability of the motion segment to move and thus sacrifices some of the ability of the spinal column to move in a natural manner. Central and lateral spinal stenosis (joint narrowing), degenerative spondylolisthesis, and degenerative scoliosis may all result from the abnormal mechanical relationship between the anterior and posterior column structures, and induce debilitating pain.
Still another therapeutic treatment of the facet joint, known in the prior art, involves an artificial facet joint where the inferior facet, the mating superior facet, or both, are covered with a cap, i.e., over (substantively all of) the facet. While potentially viable, the capping of the facet has several potential disadvantages. If the facet joint is osteoarthritic, a cap will not remove the source of the pain. Moreover, at least with use of caps over osteoarthritic femoral heads, the capping of articular bone ends has sometimes experienced clinical failure by mechanical loosening. This clinical failure is believed to result from the disruption of the periosteum and ligamenturn teres femoris, both serving a nutrition delivery role to the femoral head, thereby leading to avascular necrosis of the bony support structure for the cap. It is likely that corresponding problems could develop from capping the facet. Another potential disadvantage of facet capping is that in order to accommodate the wide variability in anatomical morphology of the facets, not only between individuals but also between levels within the spinal column, a very wide range of cap sizes and shapes is required.
Thus, there is an unmet need for additional therapies applicable to facet joints to stabilize and augment the facet joint to alleviate problems without initial resort to the more radical therapies of replacing the facet joint with a prosthetic joint or fixation of the facet joint and the inherent loss of natural movement of that motion segment.